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Tribology Transactions Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/utrb20 Crystal Chemistry and Solid Lubricating Properties of the Monochalcogenides Gallium and Selenide Ali Erdemir a a Energy Technology Division , Argonne National Laboratory , Argonne, Illinois, 60439 Published online: 25 Mar 2008.

To cite this article: Ali Erdemir (1994) Crystal Chemistry and Solid Lubricating Properties of the Monochalcogenides Gallium Selenide and Tin Selenide, Tribology Transactions, 37:3, 471-478, DOI: 10.1080/10402009408983319 To link to this article: http://dx.doi.org/10.1080/10402009408983319

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ALI ERDEMIR (Member, STLE) Energy Technology Division Argonne National Laboratory Argonne, l llinois 60439

The interatomic array and bond structure in crystalline states of crystallize as layered structures and have fairly high melting the monochalcoge~~idestin selenide and gallium selenide are de- points of 861°C and 960°C, respectively. SnSe and GaSe are scribed and correlated wit11 their solid lubricating capacity. Friction known as sandwich in solid-state physics (I), tests assessing their solid lubricating performance were carried out (2), and have been studied rather extensively by physicists on a pin-on-disk machine. Specifically, large crystalline pieces of in the past. However, their tribological properties and po- tential uses in friction and wear applications have not yet each inorganic solid were cut and cleaved into pat squares and been explored in much detail. As will be elaborated later, subsequently rubbed against sapphire balls. In another case, fine SnSe represents a group of layered compounds that also powders of gallium selenide and tin selenide ruere manually fed comprise SnS and the sulfides and of germanium, into the sliding interfaces of 440C pins and 440C disks. For the whereas GaSe belongs to a class that also includes the lay- specific test conditions explored, friction coefficients of the sapphire1 ered GaS and the sulfides and selenides of indium. galliuna selenide and sapphireltin selenide pairs were approximately Previous research on solid lubrication has largely con- 0.23 and approximately 0.35, respectively. Thefriction coefficients centrated on transition-metal dichalcogenides- and dem- of 440C pinl440C disk test pairs with gallium selenide and tin onstrated that among numerous kinds, only a few are ca- selenide powders were approximately 0.22 and approximately 0.38, pable of imparting low friction to sliding tribological respectively. For compa?-ison, a number of parallel friction tests interfaces. For example, it was found that despite their lay- were also performed with MoS2 powders and compacts, and the ered crystal structures, NbS2, TiS2, VS2, TaS2, etc., are not results of these tests are reported. The friction data, logether with as lubricious as MoS2 or WS2 (3),(4). Based on the molecular the crystal-chemical knowledge and electron microscopy evidence, orbital and valence bond theories, Jamison proposed the supported the conclusion that the solid lubricating capabilities and following explanation for the poor lubricating performance Downloaded by [RMIT University] at 01:51 13 March 2015 of NbS2, TiS2, VS2, TaS2, etc. (4),(5): in the layered crystal lubrication mechanisms of these solids are closely related to their structure of these solids, there is a region of negative elec- crystal chemistry and the nature of their interlayer bonding. trical charge that not only concentrates above the atoms of a given layer but also extends well into the pockets KEY WORDS between the chalcogen atoms of neighboring layers. Be- cause the bottoms of the pockets are positively charged (due Solid Lubrication, Friction to exposed cores of the surrounding atoms), an elec- trostatic attraction exists between the layers, making the INTRODUCTION layers of these solids shear with difficulty. As for the excellent solid lubricating capacities of MoS2 This paper describes the interatomic array and bond - - and WS2, the region of negative electrical charge is con- structure of the monochalcogenides tin selenide (SnSe) and tained within the layers. Thus, the surface of the chalcogen gallium selenide (GaSe), and correlates this knowledge with atoms is positively charged, creating an electrostatic repul- their solid lubricating performance and mechanisms. Like sion between the layers, and making interlayer slippage ex- the transition-metal dichalcogenides (e.g., MoS2, WS2, and WSe2 which are well-known solid lubricants), SnSe and GaSe ceedingly easy (4).Relatively larger interlayer separation in MoS2 and WS2 crystals is thought to result from the same electrostatic repulsion between successive layers. In general, these and other studies have supported. . the conclusion that ~meentedas a society of ~rlbologistsand Lubrlcatlon Engineers paper at the STLUASME Triboiogy Conference In not all layered structures are good solid lubricants (4). The New Louisiana, Pctober 24-27, 1993 type and magnitude of interlayer bonds are also critically Flnal manuscript approved July 23, 1993 important. 47 1 Although GaSe and SnSe have attracted a great deal of As illustrated in Fig. 1, the structure of GaSe consists of attention in solid-state physics for microelectronics and op- layers that can be regarded as a two-dimensional sheet of tical applications (I), (2),their lubrication mechanism and closely packed and strongly bonded Ga and Se atoms in the potentials for tribological applications have remained rel- sequence Se-Ga-Ga-Se (I),(lo), (12), (15). Each layer con- atively unexplored. In a set-ies of studies by Boes (6),Boes sists of four atomic planes. The Ga atoms are paired to form and Chamberlain (7), Kiparisov et al. (a),and Gardos (9), the two atomic planes inside, while the Se atoms form the the tribological and thermal oxidation properties of a com- top and bottom planes. The interatomic bonding within the posite lubricant consisting of indium/gallium'and WSe2 were layers is strong and mainly of the covalent type, whereas reported. The principal goal of these studies was to achieve the bonding between adjacent layers is weak and of the van better oxidation resistance on WSe2 by alloying it with low- der Waals type (I),(lo), (12). Because of such strong an- melting point indium and gallium. Upon curing the com- isotropy in bonding, solid crystallites of GaSe can easily be posite mixture at high temperatures, the investigators found cleaved along the basal planes. that both indium and galli~~~nunderwent chemical reaction According to Zallen and Slade (12),the structures of Gas with WSe2 to form the selenides of these metals. They also andlor GaSe are closely related to that of MoS2. In fact, demonstrated that compared to the parent WSe2, the new removal of one layer of Ga atoms in the GaSe crystal struc- composite lubricant exhibited superior oxidation resistance ture, shown in Fig. 1 produces the exact over a wide range of temperature. Also, the lubricating of MoS2 illustrated in Fig. 2. Table 1 presents the unit cell capability of this new lubricant was much superior to that of WSe2 alone, especially at elevated temperatures (8). More detailed studies by Kiparisov et al. (8)and Gardos (9) elaborated the mechanism of the improved oxidation resistance of the indiun11galliumlWSe~lubricant. According to Gardos, a protective film resulting mainly from the pref- erential oxidation of substoichiometric InSe was primarily responsible for the superior oxidation resistance of this new lubricant. The protective film was thought to effectively shield the lubricating entities against oxidation. The primary goals of this paper are to describe in detail the crystal chemistry of the monochalcogenides GaSe and SnSe, to ascertain their lubricating capabilities, and to com- bine crystal-chemical knowledge with lubrication perfor- mance to elucidate their solid lubricating mechanisms. The knowledge gained through this study can help enhance the general understanding of the lubricating mechanisms of inorganic solids with layered structures.

Ga 0 Se CRYSTAL CHEMISTRY AND BOND STRUCTURE Downloaded by [RMIT University] at 01:51 13 March 2015 GaSe (a) GaSe has been reported to exist as a layered structure in four crystallographically distinct polytypes, i.e., P-, E-, 6, and y-GaSe (lo),(11). Each polytype results from the stacking of layers in different ways. Specifically, the different co- ordination symmetry of the metal atoms in a layer and the dissimilar stacking order of each layer in the unit cells are different in each polytype (10).The P-, 6- and E-types have a layered hexagonal crystal structure (I),(lo), (12), whereas the y-type crystallizes in a rhombohedra1 structure (13). Because of their lamellar structures, all the crystalline polytypes mentioned above may have some solid lubrication capabilities. In particular, P-GaSe has a crystal structure and bond characteristics similar to that of MoS2, a popular solid lubricant (lo),(12), (14). Therefore, it has the potential to impart low friction to sliding interfaces and this paper will focus on the crystal chemistry and tribology of a P-GaSe. For brevity, the chemical symbol GaSe will refer to the P-GaSe (b) in the following sections. Micro-laser Kaman spectroscopy Fig. 1-(a) Schematic Illustratlon of layered-hexagonalstructure of GaSe. was used to verify that the GaSe material used in this study (b) lnteratomlc configuration and bonding in a layer of GaSe. Black was P-Case. circles represent Ga atoms. Crystal Chemistry and Solid Lubricating Properties 473

bonding with respect to the intralayer bonds. For MoS2 and GaSe crystals, the interlayer-to-intralayer bond length ratios were estimated to be 1.5 and 1.6, respectively (12). This suggests that MoS2 and GaSe are structurally similar and have comparable layerlike characteristics. Within the rigid two-dimensional layers of MoS2 crystals, the S atoms have a trigonal prismatic coordination around the M; atoms, shown in Fig. 2b (14). The Ga atoms in the GaSe structure are tetrahedrally coordinated as shown in Fig. I(b) (10).

SnSe SnSe has an orthohombic crystal structure consisting of two-dimensional sheets of strongly bonded Sn and Se atoms (2), (16)-(18). As illustrated in Fig. 3, the unit cell of SnSe consists of eight atoms and is two layers thick. The Sn and Se atoms are joined with strong heteropolar bonds to form the crystalline layers made up of two planes of zigzag Sn-Se- type chains (2). Each atom in a layer is connected to its near neighbors with six heteropolar bonds. The three strongest bonds, de- noted in Fig. 3 with solid lines, are established with the three nearest neighbors lying in the same layer. Two weaker bonds, denoted in Fig. 3 with broken lines, are with the more dis- tant neighbors, again in the same layer, while the last and weakest bond is formed with an atom in the adjacent layer. Hence bonding between layers is a combination of van der Waals and weak electrostatic attractions. The cell parame- ters of the SnSe crystal structure are given in Table 1 (2), (161, (17). As is clear from the foregoing, the crystal structure and bond characteristics of GaSe are similar in nature to those of MoS2. Also, the interlayer bonding in GaSe (and MoS2) across the cleavage plane is between like atorns, i.e., Se---Se (and S--4). Mainly because of a similar crystal structure and interlayer bonding, it is plausible that GaSe can also have some lubricating capability, possibly comparable to that of MoS2. For SnSe, however, the relatively stronger interlayer bonding may prevent this compound from being as lubri- Fig. 2-(a) Schematic illustration of layered-hexagonal structure of MoSa. (b) Interatomic configuration and bonding in a layer of MoS2. Black cious as MoS2 or GaSe. Also, the interlayer bonding in SnSe Downloaded by [RMIT University] at 01:51 13 March 2015 circles represent Mo atoms. across the cleavage plane is between unlike atoms, i.e., Sn---Se as shown in Fig. 3. parameters of GaSe for comparison, and the cell parameters of MoS2 are also included. The interlayer-to-intralayer bond- EXPERIMENTAL DETAILS length ratio for a given layered solid is reported to be a crude but revealing structural measure of the layerlike char- Toelucidate the lubricating capabilities of GaSe and SnSe, acter of that solid (12). In general, the greater the interlayer- a series of rudimentary friction tests was performed with to-intralayer bond-length ratio, the weaker the interlayer the flat pieces and powders of these solids on a conventional

TABLE1-UNIT CELLPARAMETERS OF SnSe AND GaSe CRYSTALSTKUC-I'URE. FOR COMPARISON,CELL PARAMETERS OF MoS2 AREALSO INCLUDED(Z), (lo),(12). (14), (16)-(18) SnSe GaSe MoS2 Cell Dimensions (nm) a 0.446 0.374 0.316 b 0.419 - - c 1.157 1.588 1.229 Interlayer chalcogen- chalcogen spacing 0.34 0.38 0.347 Unit cell Orthorhombic Hexagonal Hexagonal ? ,\ \\ \ \ \\ \ \

Orthorhombic unit cell Load (N) 2 and 5 Sliding velocity (mm.s- ') 1.7 to 5 Relative humidity (%) 8 to 27 1 Number of revolutions 500 to 1200 Cleavage 1 Ambient temperature ("C) 23 51 Number of tests 2 to 3 planes

imately 2.5 cm and fine powders of approximately 10-20 pm size. The compacts were fabricated from the powders by cold pressing at approximately 35 MPa. It is conceivable that this type of unidirectional cold-pressing may result in some degree of preferred orientation in the final compacts, Flg. 3-Schematlc lllustratlon of layered-orthorhombic structureof SnSe. and this may affect the frictional behavior of sliding test Large circles represent Se atoms, small circles represent Sn atoms. pairs. The Vickers hardness values of the 440C pin and disk specimens used in the friction tests were about 7.8 GPa. The pin-on-disk machine. The primary purpose of these friction surface finish of 440C disks was approximately 0.1 pm center- tests was to explore the solid lubricating capabilities of GaSe line-average (CLA), while that of the pin was approximately and SnSe and to establish model relationships between their 0.02 pm CLA. One end of each pin was rounded to a radius crystal chemistry and frictional performance. More detailed of curvature of 127 mm. The sapphire balls were 6.35 mm tests aimed at elucidating the possible roles of several test in diameter and had a Vickers hardness value of approxi- parameters (e.g., load, speed, distance, humidity, temper- mately 22.5 GPa. The surface finish of these balls was better ature) in the lubrication performance of these solids were than 0.01 pm CLA. Two to three tests were run under each not performed in this initial study. However, the scope of condition to verify the reproducibility of the friction data. work was expanded to include MoS2 as a reference material The results were quite reproducible. The deviations from and to provide a basis for comparison with the friction test the mean friction values were in the range of +-5% and results from GaSe and SnSe. k 1076, depending on test pairs and contact load. As mentioned earlier, GaSe exists as a layered structure in four crystallographically distinct polytypes, P-, E-, 6, and RESULTS AND DISCUSSION y-GaSe (lo), (11). The specific GaSe material used in this study was characterized by micro-laser Raman scattering The Raman spectrum of the GaSe material used in this experiments performed with an Ar ion laser. The Raman study reveals five strong Raman lines centered at 17.99, spectrometer was equipped with a photomultiplier tube as 57.99, 132.99,210.99, and 305.99cm-', as shown in Fig. 4(a). the detector and photon counting electronics. The laser As is clear, this spectrum is very similar to the Raman spec-

Downloaded by [RMIT University] at 01:51 13 March 2015 light (487.9 nm at 50 mW) was focused on the sample through trum of P-GaSe, as shown in Fig. 4(b), reported by Wieting an optical microscope to a spot size of approximately 1.5 pm and Verble (20). This confirms that the Case material used with a lOOX objective in a strict 180" backscattering geometry. in this study was P-type. Two types of friction tests were performed with the solid Figure 5(a) shows, as a function of sliding cycles, friction GaSe and SnSe materials. In one case, fairly large crystalline coefficients of the 440c1440C test pairs including GaSe and pieces of GaSe and SnSe were first cleaved into flat pieces, MoS2 powders at sliding interfaces. Initially, the friction and then, cut into squares with typical dimensions of ap- coefficients of these test pairs were comparable, e.g., ap- proximately l.5 cm x 1.5 cm, and 1 to 2 mm thick. Ac- proximately 0.22 to 0.23. During successive sliding cycles, cording to the technical specifications of the commercial they increased slightly. The friction coefficient of a test pair source (19), the solid lumps of SnSe and GaSe were 99.98 including GaSe powders eventually reached a steady-state wt.% pure. The squares were then firmly attached to a 5-cm- value of 0.24 after about 150 revolutions, whereas the fric- diameter steel disk and rubbed against sapphire balls in a tion coefficient of a test pair including MoS2 powders de- pin-on-disk machine under the sliding conditions given in creased slightly with increasing sliding cycles, and stabilized Table 2 and in the figure captions. In another case, fine at approximately 0.22. powders of GaSe and SnSe with a particle size of approx- Figure 5(b) shows friction coefficients of test pairs of sap- imately 50-100 pm were manually fed into the sliding in- phire1GaSe flat and sapphireIMoS2 compacts. Initially, the terfaces of 440C pins and 440C disks. The pin-on-disk ma- friction coefficients of sapphire1GaSe and sapphire/MoS2 chine used in this study was calibrated by applying incremental pairs were quite different. As is evident, the friction coef- forces to the pin holder in the tangential direction with the ficient of the sapphire/MoS2 pair was initially approximately increments ranging from 0.2 to 0.5 N, depending on nor- 0.3, but decreased steadily with sliding cycles and reached mal force, i.e., 2 N or 5 N. The MoS2 test material included a steady-state value of approximately 0.25 after about 200 disk-shaped-compacts with a nominal diameter of approx- revolutions. Conversely, the initial friction coefficient of the Downloaded by [RMIT University] at 01:51 13 March 2015 I'his rliscrcp;~ncv III;IV 11;lt.c been ~IIC to tlir ~lifferrnccill tllr pt~ritvi111d 1111. ~l~oisturecontent 01 thr hlnS1 usetl ill thiq ttncl ~~tl~~~~st~~~lics.'1'11~MoS? lt1l>ri~1111 IISCtlst;lnt at approui~~i;~tclv0.38 tlirt~t~gl~- ~IIIIthc~ slid it^^ tn, t~Io\ve\,er, IIICl'rictio~~ c~efficie~lt of' :I s;~ppl~irch;iIl ;rg;ti~~stSIISC was initi:~lIv;~pproxi~~iatc~l\ 0.23, 11111 t~c~ntt~;~llvi~~crcawrl to approxir~~;~tcIv ll.:3.r~ after lill rrv- oltttio~~s.tvhrrr it rc,tn;~inecl conslalit 11n1ilthe sliding rrst ~~nrlc~l.21s sl~o~vn in l:ig. fi(h). It is ir~ll)ol~litnlto 11111r tllill Iltr,sr frictio~~v;alt~cs inre significantlv l~ixl~crthzn tliosc ;I[-

I 0 0 200 400 600 800 1.000 1.200 1.400 Number of Revolutions Downloaded by [RMIT University] at 01:51 13 March 2015

01 I 0 200 400 600 BOO 1.000 1.200 1.400 Number of Revolutions

Fig. 6-Vadatlon of lrlcllon coefficients as a luncllon ol sliding cycles. (a) 440C pini440C disk test pairs lubricated with SnSe pwders (load = IN, relative humidity = 27 percent) (b) sapphire balliSnSe flat (load = SN, relative humidity = 8 per- Fig. 7-SEM mlcrgraph of transter layer lormed on a 440C pin sultace cent, temperature = 23%. rlldlng velocity = 1.7 mm.s-', ra- during sliding against GaSe-lubricated 440C disk (load = 2N. dius 01 CUNatUre 01 440C pin = 127 mm, radius of sapphire temperature = 23°C. sliding velocity = 5 mm-s', relaflue hu- ball = 3.17 mm) midity = 14 percent. radius of curvature of 440C pin. 127 mm). Fig. 8-SEM micrographs, showing platelike nature of GaSe and some Fig. %SEW mlerqlrsphs. revealing platelike nslure of SnSa and soma evidence of intercrystallite slip between platelike crystallites. evidence of inlercrystallite slip between platelike erystslllks. (a) free standing GaSe particles (a) free-standing SnSe particles

Downloaded by [RMIT University] at 01:51 13 March 2015 (b) friction tested GaSe particles (b) friction tested SnSe particles

MoS? stroctlzrr. Under the test cnnditio~isr~plored. thr lriclion cneflicients ol sliclinp intrrfiiccs illi~lin- ch~clrMoS? and (;;tSr are ralllrr cr~rnpar;~hlr. 3. 1)rspite its layerrd structure, SnSr is not ;ts It~hl-icious as XIoS? or GaSr. This ma\ hare heen due to the fact ha^ in the crys~als~ructure of SnSc. adjarel11 );I\-rrs arc hr,ond to r;a-11 other wi~h;I cornhirr;~tion 01' van rler Waals anrl long-rangr elcctrnstatic atll-aclions, wherras in MnSr ;~nrlGaSe cr)sl;~li. the ir~trrl;~vrrat- tractinn was prim:l~il?of the van ller \Vall? tvpc. 4. Fundamentallv, it i?hypothesi/rtl that thv v,liOII